Importing old unsubscribe lists

Yes, You can import them by email address only if you like to a designated list, then select all members, do a data value change and change unsubscribe to true. I would typically do this again and change the unsubscribe reason as well and note where the list came from and why the change.

You can do this faster by creating a CSV:

Email Address | Unsubscribed | Unsubscribed Reason

then fill in email, 1, and reason name

There's no reason to do this if your Marketo never had these people anyway.